Method of producing a screening smoke with one-way transparency in the infrared spectrum

ABSTRACT

The invention relates to a method of producing a screening smoke which is one-way transparent in the infrared spectrum (780 nm-14.0 μm) and opaque in the visible spectrum. According to the invention a known pyrotechnic screening smoke which is highly absorbent in the visible spectrum (380 nm-780 nm) is generated in the form of an aerosol, pyrotechnic scattered particles between 10 and 100 μm in size are simultaneously produced in said aerosol, and the resulting two-component smoke is irradiated by an infrared radiation source (spectrum: 780 nm-14.0 μm) from the smoke producer side.

The subject of the present invention is a process for the production ofa screen smoke one-sidedly transparent in the infrared spectral range,whereby scattered particles of suitable size introduced into an aerosolare impinged against by means of infrared radiation so that there isgiven a strongly marked forwards scattering on the scattered particles.The aerosol itself consist of known screen smoke strongly absorbing inthe visible range.

In the case of military deployment and also in the case of policedeployment against barricaded prepetrators, is of considrable advantagewhen, for a short time, ones own change of position cannot be observedby the opponents. Since today an observation takes place not only in thevisible range but also via IR and radar technology, in the pastsmoke-producing mixtures have been developed to a large extent which arebrought as thrown bodies between ones own position and that of theopponent and there produce a local wall of smoke which slowly breaks upin the air or is driven away by the wind or are burnt in so-called smokepots, whereupon the smoke cloud produced is spread out with the windbetween ones own position and the position of the opponents (cf. EP 0106 334 A2, DE 43 37 071 C1, DE 40 30 430 C1). Although such smokescreens give a very good protection not only in the visual but also inthe infrared spectral range, they have the disadvantage that during thetime in which the smoke is impenetrable (usually about 20-60 seconds),not only the smoke producer but also the opponent can change theposition so that for a subsequent use not only the opponent must againascertain his own position but one must oneself again also ascertain theposition of the opponent. The smoke producer would, therefore, have aconsiderable tactical advantage when, during the effective phase of theartifical smoke, he could admittedly camouflage his own actions but, atthe same time, could also follow the actions of the opponent and reactthereon.

Therefore, the task forming the basis of the invention is to develop aone-sidedly transparent screen smoke.

The known screen smokes usually consist of aerosols of solid and liquidparticles, whereby the size of the individual particles lie in the orderof magnitude of the wavelength of the radiation to be weakened so thatthey are suitable for a scattering and absorption of the light.

From U.S. Pat. No. 5,682,010 is known a one-sided camouflage action inthe visual range in the case of which such a mist cloud containing anabsorbing aerosol is simultaneously produced with an aerosol cloud ofparticles which do not absorb the light but merely scatter, whereby theabsorbing cloud in closer to ones own position and the scattering cloudto that of the opponent. In this way, the light coming from the opponentis less weakened than the light from ones own object observable by theopponent so that, in all, a residual light can be observed sufficientfor the ascertainment of the opponent's position. Insofar as both mistclouds mix with one another, the effects for both sides are the same sothat the above advantage is lost. It is a disadvantage of this devicethat the simultaneous production of the two mist clouds at definiteintervals from one another and to the discharge and target is difficultand, due to different local wind influences, the mist clouds are alsoadditionally displaced against one another. Therefore, this manner ofprocedure is not suitable for practical use.

According to DE 196 01 506 A1, a one-sidedly permeable sight barrier isthereby achieved in that one brings to shining a per se transparentartificial mist, consisting of aerosol particles or gases, by radiationwith electromagnetic radiation of appropriate wavelength (fluorescence,Raman scattering, diffuse reflection). Since this lighting up is anisotropic effect, i.e. also takes place on the side of the mistproducer, a pulsed radiation source is used, the impulse frequency ofwhich is adapted to the period of time of the emission effects.

By means of a closure, the detector of the mist user is shut off duringthe radiation time go that only electromagnetic radiation is detected inthe radiation pauses. The radiation frequency is typically so high thatthe opponent sees a continuously emitting mist cloud. In order toprevent countermeasures of the opponent, the impulse sequence of theradiation source is modulated by an algorithm not known to the opponent.The disadvantages of this process are, on the one hand, the devicesnecessary for the laborious, expensive and susceptible exciting anddetection process and, on the other hand, the toxicologically hazardousfluorescing substances in the mist cloud necessary for the radiationexcitation.

Because of the discussed disadvantages (function of the one-sided visionbarrier only in the case of ideal wind conditions not occurring inpractice; requirement of a laborious and expensive detection process orpresence of toxicologically hazardous substances in the aerosol cloud),neither of the two processes have hitherto been used in practice.

The invention solves the above-described problems in that there isproduced a smoke one-sidedly transparent in the infrared spectral rangewith the features of the main claim. The solution is promoted by themeans described in the subsidiary claims.

The producer of this smoke can carry out the detection of the opponentduring the effective phase by means of suitable electronic sids (IRcamera), whereas the sight not only in the visual but also in theinfrared spectral range is removed from the opponent by irradiation ofthe LOS (line of sight).

The present invention uses a per se known smoke, impenetrable in thevisual spectral range (λ=380 nm-780 nm) but transparent in the infraredspectral range (λ=780 nm-14.0 μm), from an aerosol with particle size of0.1-5 μm which contains additionally produced scattered particles of asize of 10 to 100 μm. This two-component smoke is irradiated with an IRradiation source from the side of the smoke producer.

In FIG. 1 is to be seen a schematic illustration of the configuration.For both sides, the visual spectral range is covered by the first smokecomponent 6. The irradiation with electromagnetic waves in the IR range,which is made available either by a high capacity lamp with appropriatefilters or by means of a pyrotechnic radiator 2, brings about, in thecase of the second smoke component, the produced scattered particles 5,a characteristic forwards scattering 7 of the IR radiation in thedirection of the opponent 9, whereas the scattering back portion of theIR radiation remains negligibly small.

The so resultant irradiation in the direction of the opponent 9 preventsthe observation of the smoke producer 1 by means of an IR camera(typical detection wavelengths: 8.0-14.0 μm), whereas with the IR cameraof the smoke producer 3, the observation of the opponent 9 is possiblewithout problems.

In order to make clear the physical effects of the scattering of the IRradiation on the produced scattered particles 5 or the aerosol particlesof the smoke components 6 covering in the visual spectral region, therewere calculated radiation diagrams according to Mie's scattered lighttheory. In contradistinction to the Rayleigh scattering, in the case ofknowledge of the optical and geometric properties of the scatteredparticles (complex refractive index m (λ); size parameter (x), thistheory offers exact solutions for isotropic spheroidal scatteredparticles on any desired size.

Since most observation apparatus work in the wave-length range of8.0-14.0 μm, as reference wavelength there was chosen λ=10.0 μm.

As example for the size-adapted scattering centres, there is used aspheroidal-shaped quartz particle with a radius of r=20 μm, wherebythere is given the size parameter x of 12.57. The wavelength-dependentcomplex refractive index amounts to m(λ)=2.67-0.05 i for λ=10 μm. Thequartz particle is present in the centre of the polar diagram in FIG. 2.The incident electro-magnetic wave coming from 180° is scattered. Thereis plotted the phase function P which is given as arithmetical middlevalue of the scattered light intensity l of the wave polarisedvertically to the scattering plane and scattered light intensity 1₂ ofthe wave polarised parallel to the scattering plane. One recognises theextremely marked forwards scattering and the negligible intensity of thelateral or backwardly scattered parts.

Therefore, scattered particles with a radius of 5-50 μm, i.e. a size of10-100 μm, are especially suitable for such an anisotropic scattering ofIR light. Since it is only a question of the scattering size and not ofthe chemical composition, solid particles were preferably used which arenot toxic or irritating to the respiratory tract and are envirnmentallycompatible. Quartz or glass meal, organic or inorganic salts areespecially suitable.

In order to demonstrate the scattering effect of the IR radiation on thecloud Components l, i.e. the aerosol particles, there are used data of atypical aerosol particle of a smoke exclusively effective in the VISregion, consisting of red phosphorus, potassium nitrate and ammoniumchloride for the scattered light analysis. After the burning, these formwith the atmospheric moisture fine droplets which absorb the VIS light.

In the case of an assumed relative atmospheric moisture of 50%, theparticle radius amounts ot 0.27 μm, i.e. the size parameter x amounts to0.17. The complex refractive index for λ=10 amounts to m(λ)=1.63-0.69 i.

FIG. 3 shows the corresponding radiation diagram. There is present analmost isotropic intensity distribution. The intensity of the scatteredelectromagnetic wave is smaller by two powers of magnitude than in thecase of the quartz particles, i.e. in the case of irradiation with an IRlight source, no one or two-sided cross-fading occurs.

The action factor of the scattering Q_(sca) is defined as the ratio ofoptically-effective particle surface, the scattering cross-sectionC_(sca), to the geometric cross-sectional surface of the particle (inthe case of spheroidal particles there applies Q_(sca)=C_(sca)/πr²), is,in the case of the chosen wavelength of λ=10.0 μm, in the case of quartzparticles greater by the factor 10⁴ than in the case of the aerosolparticles of the smoke component l. Thus, the quartz particle producesan efficient and strongly directed scattering radiation of the incidentelectromagnetic wave in the direction of the opponent.

In order to achieve a complete camouflaging of the target object withregard to the heat image apparatus of the opponent, the difference ofthe radiation intensity of the target object and the radiation intensityof the background of the position of the detector must sink below athreshold value dependent upon the particular heat image apparatus. Forthe quantitative assessment of the detectability of the target objectwith the help of the IR camera of the opponent, one uses the contrastfunction c(r) dependent upon the distance r which is defined as$\begin{matrix}{{c(r)} = \frac{{l_{t}(r)} - {l_{b}(r)}}{l_{b}(r)}} & (1)\end{matrix}$

whereby l_(t)(r) represent the intensity of the target at the distance rand l_(b)(r) the intensity of the background at the distance r. Thecontrast detectable without attenuation by atmosphere or artificialaerosols is given by: $\begin{matrix}{{c(O)} = \frac{{l_{t}(O)} - {l_{b}(O)}}{l_{b}(O)}} & (2)\end{matrix}$

The intensity of the target object at the distance r amounts to

l _(t)(r)=l _(t)(O)·T(r)+l _(p)(r)  (3)

whereby T(r) is the transmission at the distance r and l_(p)(r) is thesum of the intensity radiated into the LOS (e.g. forwards scattering onaerosol particles). Correspondingly, for the intensity of the backgroundat the distance r, there applies:

l _(b)(r)=l _(b)(O)·T(r)+l _(p)(r)  (4)

With equation (3) and equation (4), for the contrast function c(r) thereis given: $\begin{matrix}{{c(r)} = \frac{c(O)}{l + {\left\lbrack {{l_{p}(r)}/{l_{b}(O)}} \right\rbrack \left\lbrack {1/{T(r)}} \right\rbrack}}} & (5)\end{matrix}$

The effectiveness of the invention is to be made clear by the followingExample:

For a typical scenario (distance mist producer—aerosol cloud: 40 m;distance aerosol cloud—opponent: 1000 m; depth of the aerosol cloud: 8m) in FIG. 4 is illustrated the course of the contrast function c(equation 5) in dependence of the intensity relationship of theintensity l_(p) beamed into the LOS to the background intensityl_(b)(O). Not only the absorption by the atmosphere but also by theaerosol cloud was taken into account in the calculation of thetransmission T(r).

The contrast threshold C_(crit) in the case of which in the heat imageapparatus the target object is no longer to be differentiated from thebackground amounts typically to 0.35, the contrast without attenuationamounts to 1.35.

As is to be seen, the contrast in the case of a relationship ofl_(p)/l_(b)(0)λ2 sinks below the threshold value of 0.35, i.e. thetarget object is no longer detectable by the heat image apparatus.

With the help of the Mie theory, there can be calculated the portion ofthe forwardly scattered radiation by the introduced scattered particles.In the case of the above-given relationships, of a concentration of thescattered particles of 0.3 g/m³, of a wavelength of λ=10 μm and theassumption that l_(p) is given by the forwards scattering of thescattered particles, the intensity of the IR radiation source of thesmoke producer must be greater by the factor 30, for safety reasons by30-100, than the intensity of the background in order to go the contrastthreshold. If one specifies for the radiation intensity of thebackground l_(b) in the wavelength range of 8.0-14.0 μm and an ambienttemperature of 293 K a value of 40 W m⁻²sr⁻¹, the intensity of the IRradiation source of the smoke producer in this wavelength range mustreach a capacity of at least 1200-4000 W m⁻²sr⁻¹ in order that thecontrast in the heat image of the opponent falls under the contrastthreshold and thus no detection of the target object is any longerpossible.

LIST OF REFERENCES

1. smoke producer

2 IR radiation source

3 IR camera of the smoke producer

4 smoke projectile

5 size-adapted scattered particles

6 smoke components acting in the VIS range

7 forward scattering of the electromagnetic wave

8 IR camera of the opponent

9 opponent

10 backwards scattering of the electromagnetic wave

What is claimed is:
 1. Process for the production of a smoke screenone-sidedly transparent in the infrared spectral range which isimpermeable in the visible range characterised in that one a) produces aper se known pyrotechnic smoke screen strongly absorbing in the visualspectral range in the form of an aerosol and b) simultaneouslyintroduces therein pyrotechnic scattered particles the size of whichamounts to 10-100 μm and c) the two-component smoke is irradiated fromthe side of the smoke producer with an IR radiation source.
 2. Processaccording to claim 1, characterised in that, in the case of the IRradiation source, it is a question either of a pyrotechnic emitter or ofa strong-capacity lamp which is possibly equipped with appropriatefilters.
 3. Processing according to claim 1, characterised in thatparticle sizes and thus size parameters x of the produced scatteredparticles are so chosen that the effect of the strongly marked forwardscattering is given either for the whole IR range or selected particleranges within this wavelength range in the case of the IR radiation ofthe scattered particles described in claim
 1. 4. Process according toclaim 1, characterised in that the aerosol impermeable in the visualspectral range is produced by a pyrotechnic active mass based onammonium chloride, potassium nitrate and lactose.
 5. Process accordingto claim 1, characterised in that the produced scattered particles arequartz particles with a size of 20-50 μm.